Known methods of sensing process media pressure utilize a thin isolator diaphragm coupled to a transducer disposed within a pressure transmitter housing. Where process conditions require remote measurement of process media characteristics, remote seals or isolator assemblies include flexible isolator diaphragms open to the process media that transmit pressure through an incompressible fluid which is carried in suitable capillary tubing connected from the remote isolator assembly to a sensor in a pressure transmitter. In the food and beverage processing industries, sanitary process conditions are required and fill fluid for the isolator diaphragm and tubing must not be harmful to humans. Thus, if a diaphragm ruptures no material harmful to humans contacts the process media. Of course, suitable fill fluid can be any of the fluids known as Generally Recognized As Safe by the U.S. Federal Department of Agriculture, CFR Title 21.
Pressure transmitters coupled to remote isolator diaphragms provide a reliable means of measuring process pressure while preventing the process medium from contacting the internal transmitter diaphragm. Accordingly, remote isolator diaphragms sense the pressure characteristics of corrosive process media; process media containing suspended solids; viscous process media; process media of extreme temperature; sanitary or aseptic process conditions; and media that must be flushed from lines and equipment between processing batches. Sanitary processing equipment often consists of a processing tank with isolator diaphragms interposed in the tank wall interior surface. Thus, the isolator diaphragm contacts the process media and transmits pressure signals to manually accessible pressure transmitters via suitable filled tubing.
In sanitary processing applications, where contamination of process media must be avoided, periodically intense sprays of cleaning solutions scour the process media containers, including the surfaces of the isolator diaphragms exposed to the media, at times resulting in failure of the isolator diaphragm. In use, the force of the moving spray impacts the exposed isolator diaphragm surface thereby driving the fill fluid toward the joint or seal between the diaphragm and a diaphragm support member. Such stress of the diaphragm seal joint may cause metal fatigue, and this condition appears more likely for sanitary isolator diaphragms than other diaphragm assemblies because most pressure-sensing applications involve a substantially equal pressure gradient over the entire surface of the flexible diaphragm. Thus, cleaning systems used in sanitary process industries tend to degrade isolator diaphragm performance and shorten the effective life of the diaphragm.
Automated cleaning systems, termed Clean-In-Place (CIP) systems, and manual cleaning systems used in sanitary processing both create similar degradation of isolator diaphragm assemblies. CIP and manual cleaning systems operate by directing streams of cleaning solutions from nozzles at elevated temperatures and pressures. The spray jets of CIP systems systematically move in cycles over the entire exposed processing surface in a controlled manner thereby inducing deformation of the isolator diaphragm. Such severe duty increases maintenance service requirements when using the prior art diaphragms.